- Home
- Conferences
- Conference Proceedings
- Conferences
5th EEGS-ES Meeting
- Conference date: 06 Sep 1999 - 09 Sep 1999
- Location: Budapest, Hungary
- ISBN: 978-94-6282-119-4
- Published: 06 September 1999
41 - 60 of 196 results
-
-
The resistivity grid to optimize tomographic 3D imaging
Authors P. L. Cosentino, R. Martorana and L. M. TerranovaIn this paper a modified version of the old resistivity "AB rectangle" is presented. Here the potential electrodes are located along a regular grid of suitable dimensions, having a triangular or square unitary cell. The current electrodes, instead, are generally located outside of the grid (see figure below), and various directions of the transmitting dipoles may be selected so to optimize the lighting of the subsoil under investigation (as expected the results improve by increasing the number of the directions).
-
-
-
Field meauserements by using geoelectrical nullarrays on fissured limestone in the Swiss Jura
Authors S. Szalai, F. Bosch, P. Turberg, I. Müller, E. Prácser and L. SzarkaUntil now no systematical field studies have been carried out by using geoelectrical arrays, having zero potential difference over a homogeneous halfspace. We call hereafter such configurations as null-arrays. In this paper we present field results obtained over near-surface fissures in limestone by using three different null-arrays. The null-arrays were selected according to three possible types of the current distribution around the current electrodes. In the first type of the current distribution the current is propagating radially. In the second one the current lines are more or less parallel with each other. In the last one the current flow is considered as it would be of a dipole origin. The first type of null-arrays was realized by a three-electrode geometry. The second type was realized by the so called Schlumberger null-array and in the last case both dipole equatorial and dipole axial configurations were transformed into null-array geometries. The connecting line of the potential electrodes in the null-arrays are always perpendicular to that in the classical arrays. This feature made it possible to obtain a very quick joint application of these two array types and also to compare the results between the traditionally used arrays and the null-arrays. All geoelectric profiles were evaluated in a close vicinity of a quarry, where the fissures were directly seen. As we found, the null-arrays located the fissures with a higher accuracy and provided a more significant anomaly than the classical configurations did, especially in case of a thicker overburden. Since the null-arrays and classical arrays give complementary information we recommend their combined use in such near-surface studies. Besides the theoretical aspects, these field test measurements have been found to be very useful in hydrogeological applications, since fractures and fissures are the most important geological features for determining groundwater vulnerability.
-
-
-
Resistivity surveying as pre-investigation method for the hallandsas tunnel project
By T. DahlinResistivity surveying with combined profiling and sounding, also called continuous vertical electrical sounding, has been carried out along large parts of the planned railway tunnel through Hallandsås using multi-electrode data acquisition equipment. The measured data was processed by means of inverse numerical modelling. (Dahlin 1996) The first surveying was carried out over the Southern Marginal Zone in 1995 as part of complementary investigations of the rock quality (Dahlin et al. 1999). Due to the good results achieved, the technique has since been applied along most of the 8.6 km long tunnel line. In the surveys carried out in 1998 larger electrode separations, with 800 metre cable layouts, were used for increased depth penetration. Expanded measurement protocols were also employed for enhanced resolution. The results (see figure 1) display a strong variation in modelled resistivities along the line, where in particular the Southern Marginal Zone and the Möllebäcken Zone stand out as low resistive zones. Apart from these, several low resistive zones of smaller extent are evident, which can be interpreted as fractured or weathered zones. Limited parts of the tunnel stretch exhibit the high resistivities that are normally expected for crystalline rock of low fracture density below the groundwater level. The resistivity models were used, along with other available geophysical data, as a basis for creating conceptual models of the structural geology and the variation in quality of the rock. These models were used as a basis for siting test drillings, to verify and refine the conceptual models. A comparison with core drilling data shows that the resistivity results give a good overview of the structural geology and variation in relative rock quality. It was, however, not possible to correlate e.g. Q-values with resistivity on a detail level. This is partly dependent on the differences in scale and resolution of the two types of data, but there may also be a lack of a simple connection between Q-value and resistivity. It is quite possible that a better correlation could be achieved with a different parameter as measure of the rock quality. Furthermore, the resistivity models as well as drilling data and documentation from the tunnelling (Dahlin and Sturk 1998) shows a strong inhomogeneity of the rock in Hallandsås, which means that the individual boreholes are probably only representative for a very small volume in their vicinity.
-
-
-
SNMR test measurements in Austria
More LessIn December 1997 the NMR- method (NUMIS-equipment from IRIS-instruments) was tested the first time in Austria for groundwater research. NMR soundings were carried out in two different areas of Austria. The locations were chosen at sites where other geophysical methods, especially geoelectric multielectrode profiles, had been used before, to correlate the results with the findings of these methods. Unfortunately at location Pulkau the proton resonance frequency, calculated from the measured local magnetic field, was a multiple of the 50 Hz frequency. Therefore no reliable measurement was possible in that area due to the high background noise. In the area of Marz a total of 9 NMR soundings were carried out using both square loop and eight-shaped-loop. At two locations also no measurements were possible because of the high background noise of 6000 to 60000 nV. Only at the location “Marzer Kogel” reliable measurements were possible at some points, e.g the Paulinerkloster. In this area an intensive geoelectric research program was carried out for groundwater research purposes the year before. The results of the NMR sounding at the location Paulinerkloster are shown in fig.1. Here only the eight-shaped-loop gave good results. The inversion results show a constant water content of 20% between 12 and 50m. Two months before a multielectrode geoelectric profile was measured crossing the location of the NMR-sounding. The results are shown in fig.2. At the location of the NMR-sounding a high resistive area down to about 80 m can be seen. The results of both methods were so promising that in the summer of 1998 a borehole was made there. The results of the drilling fully confirmed the findings of the geophysical research. As the quantity and quality of water is sufficient, a local water supply station will be planned for the near future.
-
-
-
Detection and estimation of problem zones in salt rock with the aid of underground geophysical exploration methods
Authors J. Kulenkampff, L. Aschmann, E. Moïse, F. Jacobs, G. Kurz and B. FlucheHaving a very low permeability, a high plasticity, and a high heat conductivity, salt rocks are principally suited for deposits of toxic and radioactive waste. In Germany exist a big number of salt domes and bedded layers which could serve for underground storage of hazardous waste. However, recent tectononic fault zones and areas of high water content may bear severe risks for mining and underground deposits, which are mainly due to the high solubility of salt rocks. Therefore reliable detection of fault zones and humid areas with non destructive methods is crucial for the appraisal of underground nuclear and toxic waste deposition sites in salt rocks. A qualitative detection of such problem zones with non destructive geophysical methods is possible. But it is not yet reliable enough for legal purposes. The security appraisal of hazardous waste deposits based only on non destructive geophysical methods not seems possible yet. In the past, these methods were usually applied as stand alone methods. In the frame of a research program of the German Research Ministery (BMBF) a complex interpretation system of geophysical methods shall be developed, including seismics and ultrasonic acoustics, high and low frequency electrical methods, in order to reduce the uncertainty of the geophysical results and to approve a more quantitative evaluation.
-
-
-
An other Gis: The national geoelectric information system
Authors L. Sőrés and E. PrácserThe extensive use of electromagnetic geophysical methods in Hungary was started in the 50- ies by the Eötvös Loránd Geophysical Institute (ELGI). Tens of thousands of DC soundings have been measured, reported and closed to files since the first systems started to collect geophysical data. Long before the PC age, most of them were processed by hand, using precalculated curve albums. In the store-rooms of ELGI 40 meters of geophysical documentation was sleeping it's Rosen-bud dream and waiting for the kiss of modern information technology. In 1998 a long term data archiving project has been started in the ELGI. The aim of this project is to save the tremendous amount of geophysical information that has been collected in Hungary during the last decades, to build it into a GIS - database system and bring it back to life. The database is proposed to include all available DC and TDEM sounding information and to assure access to gravity, to ground and airborne magnetic data as well. Data archiving started with the engineering geophysical measurements carried out on and around the lake Balaton from 1974. Sorting out and reorganising the old documentation, to prepare it to uploading is a very long process with high man-power requirements, but definitely pays off, if we look at the results. In 1998 about 3500 DC soundings from that area were entered, in 1999 an other 3000 is going to be entered into the system. All these data will be available again for geophysical reprocessing and environmental analysis.
-
-
-
Stream-function used for current-lines' construction in 2-dimensional DC modeling
Authors A. A. Bobachev, I. N. Modin, E. V. Pervago and V. A. ShevninThe stream function described is employed for the presentation of 2D DC modeling results. The 2D model is understood as a 2D medium with linear current electrodes, oriented along the inhomogeneities' strike direction. In this case both the medium and the electric field depend on two space coordinates only. Modelling becomes much easier than considering point current electrodes, where the electrical field always is three-dimensional. Meanwhile the actual results of such modelling are qualitatively equivalent to 3D modelling with point electrodes, as long as the measurements are conducted across the objects. The classical modelling presentation is in apparent resistivity which reflects an electric field distribution on the earth's surface. Quite often the connection of measured anomalies with a geoelectrical model is rather complex (fig. 1, A and C). The visualization of DC current lines simplifies understanding of the electric field's structure. Current lines are used in almost each textbook, but a practical techniques for their construction is usually not included. The evident way for drawing current-lines is the step by step continuation of a line from some point along the electric field direction. The practical realization of such approach is not trivial. For a 2D field it is possible to make use of the stream-function. This function is often used in EM field modeling [flux function, Berdichevsky, 1984]. A contour map of the stream-function corresponds to the stream-line distribution. Thus the problem of current streamlines' construction is reduced to the calculation of the stream-function in the research area. This can be achieved by calculating secondary surface charges, which are determined by 2D modeling, using Fredholm's integral equation of the second type relatively of electric field [Escola, 1979]. The stream-function’s (ψ) physical definition is the difference between stream-function's values in two points in space is equal to the electric current intersecting a curve connecting them:
-
-
-
Electrical imaging over simple sediment structures
More LessElectrical imaging or tomography is a survey technique employed to build up both a laterally and vertically varying picture of the electrical properties of the subsurface. The image can therefore be related to lithology and fluid variations of the underlying sediments. The technique is now widely used in geological and resource studies, archaeological surveys and in various types of hydrogeological investigations. Electrical imaging appears to be a versatile technique and has been used here to try to determine and identify simple structures and trends within shallow Quaternary sediments.
-
-
-
Recent advances in engineering geophysics Spectral IP and Multifrequency EM
Authors E. Niederleithinger and G. PlaumannThere are two main needs to improve the usability of geophysical methods for geotechnical and environmental investigations: the speed has to be increased to optimize costs and better correlation is needed between geophysical results and true subsurface parameters. Two methods - multifrequency EM and Spectral IP - are presented. None of them is entirely new, but they are going to be used in practise now. Measurements from different sites are shown.
-
-
-
A new wide-band radiomagnetotelluric measurement system with HTC-Squid
Authors T. Radic and H. BurkhardtThe radiomagetotelluric (RMT) method estimates the resistivity distribution of soil by measuring and evaluating the em-fields of radiotransmitters in the very low-, long- and medium wave frequency range. The method potentialy does not require many personnel for field measurements. It has all the features for fast recording of complex geological structures. Presently available RMT instruments have serious limitations. Therefore, we have designed and tested the following improvements: Estimating full impedance tensor To explore complex shaped subsoils knowledge of full electrical impedance tensor is indispensible. We introduce a method to estimate all four tensor elements in the radio frequency range. This makes it possible to evaluate RMT-data by using conventional and powerful MT inversions programs. Fast speed in measurement Progress in measurement is determined by the time to move the instrument from one point to another, registration time and time necessary to process the time series of typically some ten million samples per channel. To minimize the registration and the processing time we build a very fast digital time series preprocessing electronics. It calculates impedance tensor for 16 frequencies in RMT range in parallel from one registration within a few seconds. Using superconducting quantum interference device (SQUID) magnetometer A liquid nitrogen cooled SQUID Magnetometer was tested. SQUIDs are invaluable for the near future especially if wideband registration with mobile instruments, as presented here, is required. Extended frequency range The RMT technique is often unable to penetrate highly conductive overburden like dump sites, clay layers or contaminated ground water. The reason for this is that the lowest available frequency is too high. Therefore, we have combined the RMT frequency range (10 kHz to 1600 kHz) with the CSAMT (frequency: 1 kHz to 10 kHz). This permits a threefold increase in the depth of penetration. At low frequencies we use a light battery-driven horizontaly orienteted magnetic transmitter to generate the em-field. This is necessary because of absence of radio transmitters below 10 kHz.
-
-
-
Detecting of fractured zones with geoelectric and elctromagnetic methods
More LessA frequent task in hydrogeology, environmental or engineering geology to detect fractured zones within compact rocks. The problem can easily be handled in the case of outcrops. But a complex approach is necessary when the basement is covered with conductive overburden. Penetration depth, resolution power and efficiency are the main aspects when deciding the methodology. The poster presents an example of the integrated application of electromagnetic (EM: VLF, EM-31, Protem) and direct current (DC) methods. According to the geological model, there is a varying thickness of low resistivity sediments above the resistive basement. Fractured zones are supposed to be within the compact basement. The tasks were on one side to detect the thickness of the overburden and on the other side to delineate the covered fractured zones within the basement. The resistivity of the covering sediments varied between a few 10 - 100 ohmm. The resistivity contrast between the basement and the overlying sediments is at least twice. The tectonic/fractured zones could be followed as resistivity lows.
-
-
-
In-Situ and laboratory measurements of electrical resistivity on marine sediments
Authors A. Just, K. Helbig, Ch. Flechsig, F. Jacobs and R. EndlerLaboratory measurements on core samples are commonly used to investigate the physical properties of marine sediments. But sediment properties are altered due to the removal from in-situ pressure and temperature conditions. Therefore, in-situ measurements are preferable. In a research project of Leipzig University and Baltic Sea Research Institute, an Electrical In-Situ Apparatus (ELISA) is developed which allows high resolution measurements of electrical resistivity in soft sediments in a tomografic principle. The instrument will consist of three cylindrical plastic probes (length 3 m) with 25 point-like electrodes (distance 10 cm), which are connected by multicore cables to a multichannel resistivity meter onboard a research vessel (see Fig. 1). First part of the instrument is a single probe with 25 electrodes.
-
-
-
Electromagnetic investigations for the solution of engineering and evireonment problems with use of radio stations and electrotransfer lines fields
Authors A. K. Saraev, M. I. Pertel, V. G. Ivochkin and P. M. KostkinThe features of objects and conditions of measurements it is necessary to take into account at engineering and environment investigations by electromagnetic methods with use of radio stations and electrotransfer lines fields. The devices and technique should combine activities by electromagnetic profiling and soundings methods. The most informative permit to obtain the measured field characteristics, which allow not only localizing of the conductive bodies, but also to map the features of medium structure. It is necessary for localization of the objects with small horizontal dimensions to observe with rather dense grid and distance between points 1 – 2 m. The dense grid of observations is also necessary for data interpretation, at which it is possible to take into account the distortions in separate points. Therefore, the short time and low cost of measurements for one observed point are very important. The methods should provide the study of the geoelectrical section in range of depths – from parts of meter up to several tens of meters. As the objects can be characterized by electrical resistivity, varying in rather broad range (units – thousands of ohmmeters), frequency band of used methods should vary from a tens hertz up to hundreds kilohertz. In some cases, there is the necessity of the small area study and the electrical antennae should have accordingly small sizes (no more than 2 – 4 m). It is necessary to ensure a capability of observations with ungrounded electrical antennae at asphalt, concrete or gravel cover of the site and to use the jamproof devices for realization of activities at high level of industrial noise.
-
-
-
Two-dimensional interpretation of long geoelectric apparent resistivity profiles
More LessUsing up-to-date computer-controlled multi-electrode geoelectric instruments a large amounts of measured data can be acquired relatively quickly. An appropriate example of the application is the geoelectric measurement performed along flood-control embankments where the purpose of investigation is either the status survey of the embankment, e.g. delineation of fissured or mudded zones, or the detection of buried old river-beds below embankment. In both cases it is expedient to measure densely ( regularly at every meter ) sounding curves having enough points to obtain a suitable vertical resolution. Using multi-electrode system GRM-30 developed in KBFI TRIAS Ltd., Hungary two person can measure more than 1 km long profile during one working day measuring at every meter a 30 point pole-pole curve ( AM=1-30m ). To process a such large amounts of measured data very quick and efficient interpretation procedures are needed. We compare the efficiency of four inversion procedures based on different principles. These methods are as follows: 1. Method of alpha-centers; 2. Smoothness-constrained least-squares method using finite-difference based forward calculation; 3. Deconvolution method based on Born-approximation; 4. Occam-inversion using a multigrid+finite element technique in the forward modelling. In the first three methods a flat topography is assumed but in the last case an arbitrary two-dimensional topography can be used. All procedures are applied to the same simulated and measured datasets. We present here two examples to be inverted: 1. In the Figure 1 a buried old river bed can be seen. The resistivity decrease between 5900 and 5980m is caused by an old river bed having material with high clay content( old river beds can be recognized as an resistivity anomaly having or lower or higher resistivity depending on the curvature of old river). In the Figure 2 the interpreted results of measurements performed on the top of a partially corrected dam can be seen. Corrected segments and those which have to be corrected are well distinguishable. To decrease the computational time and the amount of required memory in the second and fourth case the long profiles are split up into overlapping shorter segments before inversion. After connecting cell-resistivities obtained from the inversion of different segments often artificial anomalies occur in the real resistivity-depth sections. Therefore it seems that the most reliable results can be obtained from a such method which inverts total measured profile simultaneously. Deconvolution method is suitable for this purpose but in the regularization in the solution of relatively small but unstable, illconditioned linear system of equations there are some unsolved problems yet. We investigate the possibility of segmentation in the solution of forward problem, too. If only short geometrical parameters ( comparing to the length of profile ) are used in forward modelling of geoelectrical measurements, e.g. when pole-pole configuration is used, than it is allowable to divide the two-dimensional resistivity model into segments and to build up the calculated apparent resistivity pseudosection from the values calculated above different segments.
-
-
-
Infinite elements: A refinement on the finite-element method for 3D DC resistivity modeling
Authors M. Lajarthe, N. Florsch and J. J. SchottSolving the 3D resistivity problem is fundamental to understand the complex response in most environmental contexts. The finite-elements method is a classical tool to solve the dc equations (Coggon 1971 , Pridmore et al. 1981, Holcombe et al. 1984). The forward modelling is often performed in a limited domain, and requires to specify potential boundary conditions. Since the latter are physically unrealistic, they lead to unacceptable errors, especially when dealing with small domains. This paper presents the improvements which can be obtained by using infinite elements in the outer part of the main studied domain, as they permit to discard the Dirichlet condition to infinity.
-
-
-
Resistivity research of quaternary deposits in Livingston Island (Antarctica)
Authors A. Muñoz-Martín, J. Giner, J. M. González-Casado, J. López-Martínez, J. J. Durán, P. Gumiel and E. SerranoGravel and boulder deposits (mainly Holocene in age) of continental and marine origin, are relatively abundant in ice-free areas near the seashore in Antarctica, especially in the South Shetland Islands. The geophysical study of these unconsolidated deposits allows establishing the possible existence of permafrost and buried ice bodies, which have an environmental interest. The establishment of the internal structure of these detritic deposits, as well as the geometry of their basement, by means of geophysical methods, also allow making deductions about the possible existence of recent tectonic episodes.
-
-
-
High-Resolution TEM for electromagnetic scanning of near surface layer with moving sensors
More LessElectromagnetic scanning is a new high-resolution technology for the investigation of a nearsurface layer down to depths of a few tens of metres using a controlled electromagnetic radiation at the frequency interval from 1 kHz to 5 MHz. Over this range the distribution of electromagnetic energy is of diffusive-wave character which is very promissing for solving a wide scope of problems. Using the fundamental frequency lower that of georadars the proposed approach has no limitations connected with increased electrical conductance of a medium and depends less on near-surface inhomogeneities. Particular features of the method consist in high density of the spatial-temporal recording of the field; the possibility to study real conductive media; obtaining the distribution of electrophysical features and object positions by spatial-temporal projections.
-
-
-
Electromagnetic field propagation in axially symmetric horizontal-inhomogeneous on dielectric permeability and electrical conductivity medium with dispacement current account
Authors G. M. Trigubovich, M. E. Royak, Y. G. Soloveichik and A. G. ZadorogniyThe study of a high-frequency electromagnetic induction spectrum in ground calls today increased practical interest. This direction is actual for solving problems of environment protection and some engineering geophysics problems: permafrost mapping, searching of underground communications flooding source, prediction of potentially dangerous appearances. These tasks are connected to mastering of nanosecond range of field component recording, where it is necessary to take into account displacement currents and to understand wave and diffusion character of electromagnetic filed propagation.
-
-
-
Transient EM sounding in the study of permafrost
More LessPermafrost covers more than 60 % of the surface area in Russia. Geophysical methods, which allow studies of permafrost rock structure, composition and distribution, are extremely important. Recently in Russia the transient EM sounding method has taken the leading position with respect to other electric methods of permafrost studies and in hydrogeological and engineering applications. The high level of geological information and technological effectiveness of the transient EM sounding method are well known. However, the processes of induced polarization, superposed to the transient EM processes, distort the latter up to the change of electromotive force sing. Permafrost is included into polarization phenomenon at the first place. For the interpretation of TDEM data the most effective solution is mathematical modelling, which requires software for solution of direct problems, having high speed of execution and high accuracy in calculation. The author made calculations and created a set of programs for transient sounding processes calculation in polarization multi-layered media. We used a set of horizontal polarizing conductive plates to model layered-earth. The problem was solved using the method of serial approximation .The problem was solved using the method of serial approximations. This makes it possible to separate and analyse both induction and polarization components of EM field.
-
-
-
Processing strategies for the application of the GPR technology to humanitarian demining
More LessThe demining methods presently used by army or by civilians (i.e., dogs, metal detectors, mechanical systems, etc.) are not appropriate to ensure a rapid solution of the humanitarian demining problem. There is an urgent demand for new methods that must be faster and cheaper than the existing methods. Since there is no regular technology that seems to be selfsufficient, the EC is encouraging projects aimed to the development of multi-sensor systems. As the metal content of modern mines is nearly zero, metal detectors have to be calibrated for extremely high sensitivity. As a result, the false alarm rate of metal detectors is very high and the demining procedures very slow. GPR is a candidate sensor that might be combined with a metal detector to reduce the false alarm rate and to ensure that plastic mines are detected. In the past few years, the DETEC (Demining Technology Center) group of the Swiss Federal Institute of Technology in Lausanne performed a number of interesting experiments with a 1GHz GPR both in a laboratory and in some field test sites (Zagreb and Cambodia). We used the raw data kindly delivered by the DETEC team to explore the potential and the problems of the GPR technology and to design and test specific algorithms to improve the detection rate.
-